1. Field of the Invention
This invention relates to a polarization network for converting between linearly polarized signals and circularly polarized singals, and for changing the orientation of linearly polarized signals.
2. Prior Art
The prior art teaches a variety of ways to convert a linearly polarized microwave signal to a circularly polarized microwave signal and vice versa. For example, the transformation between linear and circular polarization can be accomplished by a septum polarizer. A septum polarizer usually is a threeport waveguide device, where the number of ports refers to physical ports. It may be formed by two rectangular waveguides that have common wide or H-plane walls. The two rectangular waveguides are transformed by a sloping septum into a single square waveguide. Various prior art septum polarizer designs are illustrated and described in U.S. Pat. No. 3,958,193 issued May 18, 1976 to James V. Rootsey, assigned to Aeronutronic Ford Corporation, now Ford Aerospace & Communications Corporaton, the assignee of the present invention.
In a septum polarizer, a linerarly polarized transverse electric field microwave signal is converted, through the action of the septum, into a circularly polarized (CP) microwave signal and vice versa. The linearly polarized signal is introduced into one of the two rectangular waveguide ports and produces in the square waveguide port a microwave signal having either right-hand circular polarization (RHCP) or left-hand circular polarization (LHCP). Whether RHCP or LHCP is produced depends upon which of the two rectangular waveguide ports is excited. It is possible and in some applications very desirable to introduce simultaneously in both of the rectangular waveguide ports linearly polarized signals to produce in the square waveguide port both RHCP and LHCP signals, or vice versa. The two linearly or circularly polarized signals may constitute separate information channels. If the RHCP and LHCP signals co-existing in the square waveguide port have perfect circular polarization characteristics, they are completely isolated from one another and there is no interference between them.
A perfect CP signal has a rotating electric field that can be regarded as the vector resultant of two orthogonal components Ex and Ey having sinusoidally varying magnitudes that are exactly equal in amplitude but 90.degree. out of phase with one another. The closer simultaneously existing RHCP and LHCP signals come to the perfect CP signal, the greater is the isolation between them. The axial ratio AR is the ratio of Ex to Ey and is an indication of the degree to which a CP signal has departed from the ideal. In dB, the axial ratio AR is equal to 20 log Ex/Ey. Perfect CP signals have an AR of 0 dB.
Another known means for generating various combinations of right and left hand circular polarized waves is a microwave switch. The polarized waves are applied to directional filters which direct the right hand waves to a first antenna system while the left hand waves are directed to another antenna system. A well known microwave switch is the Faraday rotator which includes a rotator section for rotating the polarization of the linear signal and a quarter wave plate for generating a predetermined amount of right hand and/or left hand circular polarized waves.
The rotator section is made up of a cylindrical waveguide member having an input port to which a linearly polarized wave is applied. A ferrite rod is axially suspended within the cylindrical waveguide and a coil is mounted about the outer circumference of the waveguide, coaxial with the ferrite rod. As current is applied to the coil, a magnetic field is induced in the ferrite rod which causes the plane of polarization of the linear wave to be rotated in an angle and sense or direction which is proportional to the current applied.
The linear signal having its plane of polarization rotated is then applied to another cylindrical waveguide member having a quarter wave plate therein for generating right hand and left hand circular polarized waves. Thus, the ratio of right hand to left hand circular polarized waves is determined by the current applied to the coil.
The Faraday rotator is relatively bulky and heavy, and requires continuous power to maintain a particular angle of the plane of polarization of the linear wave. In addition, the output signal of the Faraday rotator is temperature sensitive. For example, environmental temperature changes cause the permeability of the ferrite rod to vary, which in turn varies the plane of polarization of the linearly polarized wave. Thus, as the temperature changes so does the ratio of right-hand to left-hand polarized waves.
Another method of rotating the linear polarization of a wave is to use an electrically rotated quarter wave plate. This method uses a ferrite tube in a circular waveguide having a quadrupole field around the waveguide. The ferrite tube is transversely magnetized by the quadrupole magnetics to rotate the plane of polarization of the input wave. Also, an A/C motor stator arrangement may be used in place of the quadrupole arrangement for electrically rotating the magnetic field about the waveguide. The drawbacks of such an arrangement are that such a device is heavy, inefficient, and in addition requires a holding current.
The prior art also teaches that selectable left or right hand circular polarization and linear polarization can be provided by two 90.degree. polarizers. Clearly, the use of two polarizers is less desirable than being able to use only one. This is particularly true in any antenna feed or microwave application requiring selectable circular and linear polarization. These are some of the problems this invention overcomes.
Representative of the prior art are U.S. Pat. Nos. 2,858,512, 3,296,558, 3,857,112, and 4,060,781.